Steven Weinberg ’54, the theoretical physicist whose Nobel prize-winning work transformed scientists’ understanding of fundamental forces, died on July 23. He was 88.

“[His work] is basically the foundation of everything we do in particle physics, what we now call the Standard Model,” said Prof. Csaba Csaki, physics. The Standard Model explains the fundamental particles and interactions that make up the universe.

In his most significant work, the Cornell alumnus and former visiting lecturer proposed the electroweak force, which unifies the electromagnetic and weak forces, two of the fundamental forces that explain the behavior of all particles in the universe.

The electromagnetic force is the push or pull that occurs between charged particles, while the weak force is responsible for radioactive decay, according to Csaki. In his groundbreaking theory, Weinberg suggested that these two forces are identical at very high energies.

To reach this conclusion, Weinberg relied on the Higgs mechanism, which explains how the particles that carry fundamental forces and allow for the exchange of energy acquire mass.

“Weinberg took the Higgs mechanism and applied it to [develop] a theory that correctly describes nature,” Csaki said.

Weinberg predicted the properties — including the mass — of a force-carrying particle known as the Z boson in a 1967 paper. Bosons are elementary particles — particles that cannot be broken down into smaller subparts — that carry the fundamental forces.

“The funny thing is when he wrote this paper, nobody took it seriously,” Csaki said.

“For like five or six years, it had almost no citations. Then around 1973, experiments and theoretical advancements came around, and they started to show that the world is really the way Weinberg predicted.”

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This evidence included the Nobel prize-winning work of Gerardus ‘t Hooft and Martinus Veltman, who developed a mathematical theory that validated Weinberg’s prediction in the early 1970s.

As further proof of Weinberg’s theory mounted, the initial skepticism he faced began to fade. In 1979, he received the Nobel Prize in Physics for the electroweak unification, along with contributors Sheldon Lee Glashow and Abdus Salam.

Since then, physicists have continued to verify Weinberg’s theory of electroweak interactions. In 1983, the Z boson was observed for the first time, and its properties were exactly as Weinberg predicted. The discovery of the Higgs boson — another force-carrying particle — in 2012 provided additional experimental evidence of the mechanisms underlying Weinberg’s theory.

Csaki explained that Weinberg also went on to lay the groundwork for effective field theory, which describes another fundamental force, the strong interaction. The strong force is what binds an atom’s nucleus together.

Weinberg’s theoretical advances revolutionized particle physics and continue to define the trajectory of the field.

His influence can be seen in current efforts toward developing a theory that adds the strong force to his unification of the electroweak force, according to Csaki. This theory would show that all three forces are identical under certain conditions.

Beyond his Nobel prize-winning work, Weinberg contributed to some of the most important areas of modern physics, including general relativity, cosmology and quantum field theory. According to Csaki, Weinberg’s three-volume textbook on quantum field theory is considered one of the most comprehensive works on the subject.

Csaki added that Weinberg’s success as a scientist was in part due to his open-minded nature and desire to continue learning, even late in his life. Weinberg’s interest in theoretical physics stretched back to childhood — he became interested in the topic by age 16, motivated by his father’s encouragement.

“He was always broadening his view, even at an old age volunteering to teach a class that is slightly [outside] his expertise, using it to really deeply [delve] into the subject, and then actually writing the textbook on it,” Csaki said.

Weinberg was also passionate about bringing science out of the classroom and into contexts accessible to more general audiences. His 1977 book titled *The First Three Minutes* provides a look at the earliest moments of the universe.

Weinberg completed his undergraduate degree at Cornell and went on to earn his Ph.D. in physics at Princeton University in 1957.

In 2007, Weinberg returned to Cornell and delivered lectures on the history of science as part of the University’s Messenger Lecture Series, which highlights topics understandable to the general public.

Weinberg is survived by his wife Louise and daughter Elizabeth.